Described in this manual are two field investigations which utilize the natural environment to teach ecological principles to high school students. In groups of five to ten, students study two aquatic environments and then prepare a booklet which summarizes their work. The manual is divided into four parts: (1) teacher's guide; (2) student's guide; (3) rivers, lakes, and ponds; and (4) sandy beaches. The teacher's guide describes goals and objectives, prerequisite student knowledge and skills, and stages of group growth. An overview of basic ecological principles and a discussion of the field study goals comprise the student's guide. The remaining two parts provide instructions for investigating a freshwater and a marine ecosystem. These activities consist primarily of transect and quadrat studies, chemical analysis, and species identification. Included in the appendix are some field procedures and a list of references. (WB)